1. Field of the Invention
This invention relates to the repair of bone defects in the cranium or in facial or maxillofacial bones. In particular, it relates to a specific restorative combination of scaffolding and bone replacement material which is used to repair such defects, to the scaffolding itself, and to the associated methods, surgical techniques, medical devices and kits.
2. Discussion of the Prior Art
A variety of devices and materials have been used in attempts to repair bone defects. Metal plates have been employed to repair cranial defects for centuries. However, contour reconstruction of the non-stress-bearing craniomaxillofacial skeleton continues to be a technical and materials science problem. Metals are difficult to shape and are hampered by disadvantages such as infection and corrosion. Polymers are often encapsulated by scar tissue, resulting in significant rates of implant infection and/or extrusion. Autogenous bone grafts and other biological materials may cause donor site morbidity, may exhibit significant post implantation resorption, and are troublesome to accurately form to skeletal defects.
Of the alloplastic materials used to augment and replace the craniomaxillofacial skeleton, the most promising and best tolerated are calcium phosphate-based compounds. Since the mid-1970's, a variety of preformed calcium phosphate materials in the form of hydroxyapatite have been used in clinical applications within medicine and dentistry. To some extent, the applicability of these preparations has been limited because they had to be preformed as hard materials. These ceramic forms of hydroxyapatite are heated to fuse individual crystals to each other through a process called sintering. This results in a tough, functionally non-resorbable material available in dense or porous forms. See Costantino and Friedman et al., "Hydroxyapatite Cement: I. Basic Chemistry and Histologic Properties," 117 Arch. Otolaryngol. Head Neck Surg. 379 (April 1991).
Most of these implants have been in the form of prefabricated, sintered hydroxyapatite in either granule or block forms. These preparations have several drawbacks, including a limited ability to conform to skeletal defects, particularly in the case of blocks; inadequate structural integrity of granules (which do not bond together), and difficulty in modeling the implant to the shape of missing skeletal tissue with both blocks and granules. The block form of hydroxyapatite provides structural support, but among other complications, must be held in place by mechanical means, which greatly limits its use and its cosmetic results; and it is very difficult to saw a shape such that it fits the patient's individual defect. The granular form produces cosmetically better results, but has a very limited structural stability and is difficult to contain during and after a surgical procedure. All of these products are ceramics, produced by high temperature sintering, and are not individually crystalline, but rather have their crystal boundaries fused together. These ceramic-type materials are in general functionally biologically non-absorbable (having an absorption rate generally not exceeding on the order of 1% per year).
A porous, non-resorbable material based on coral allows intergrowth with bone, but ultimately becomes only approximately 20% bone with the remaining 80% subsisting as scar tissue. "HA RESORB" made by Osteogen is a form of absorbable apatite, but is not a cement and is not entirely composed of hydroxyapatite. It is granular and not adhesive. "HA RESORB" is loosely rather than adhesively packed into place. It is resorbed quickly and may result in defect formation. In the dental materials market, "HAPSET" is a composition of calcium phosphate granules and cementable plaster of Paris (calcium sulfate). This material is not truly a hydroxyapatite cement and contains too much calcium sulfate for most biological uses. The calcium sulfate component of such a composition is resorbable, but not the calcium phosphate granules.
One alternative which has been proposed is a composite formed of a biocompatible metal, such as titanium, tantalum or niobium, mixed as a powder with powdered ceramic calcium phosphates and then pressed or sintered into an implant. See U.S. Pat. No. 4,599,085 (Riess et al.).
A specific but fairly common bone repair problem in the cranial area is the repair of burr holes after a craniotomy. Burr hole defects after craniotomy can easily be detected by the naked eye shortly after an operation; they may cause retraction of the skin and are cosmetically unappealing. Thus, in modern neurosurgical operations, the repair of burr holes is an important end step to the craniotomy.
Attempts at burr hole repair have included the use of different metals such as aluminum, gold, vitallium, tantalum and stainless steel. Also, compositions of plastic, acrylic resin and ceramics have been employed. Frozen lyophilized human cadaver bone, chips of autogenous bone, and coral have been used to fill burr holes. Attempted plugging of burr holes with autologous bone plugs formed with rongeur, mostly from the temporal bone, is another technique that has been employed. All of these approaches have been unsatisfactory to some degree. Local irritation, failure to achieve good cosmetic results, additional surgical time or insufficient sources of cylindrical bone plugs have been some of the difficulties encountered. Use of a specialized burr hole saw for simultaneous production of burr holes and appropriate autologous bone plugs which can readily be reinserted and locked in place using bone dust and Tiscel glue has also been described. Bostrom et al., "Reconstruction of Craniotomy Burr-Holes with Autologous Bone Blugs sic! Made by a New Hole-Saw," 105 Acta Neurochir. 132 (1990). Examples of metal plates for burr hole repair by bridging the outer surface of the defect are the micro burr hole covering and bone flap fixation plates from Leibinger GmbH. See "Titanium Micro System," Leibinger GmbH 1992. Titanium micromesh for defect-bridging outer surface reconstruction of bony structures is illustrated in the same brochure. See also the larger scale "DUMBACH TITAN MESH-SYSTEM" ("DTM") of Osw. Leibinger GmbH (1990), which may be employed with autogenous spongiosa, granulated demineralized pyrolized bone and hydroxylapatite granulate if desired.
Recently, a new type of calcium phosphate cement that sets to hydroxyapatite in vivo has been developed. See U.S. Pat. Nos. Re. 33,221 and Re. 33,161 to Brown and Chow. This is essentially a bone replacement or bone substitute cement which can be applied intraoperatively as a paste and subsequently sets to a structurally stable implant material composed of microporous hydroxyapatite. This is a nonceramic form of hydroxyapatite cement which is produced by direct crystallization of hydroxyapatite in vivo, and does not require heating for the formation of a structurally stable implant. These new hydroxyapatite-forming cements are biologically compatible and are self-hardening to form a mass with sufficient strength for many medical and dental applications. When implanted in bone, the cement resorbs slowly and is gradually replaced by new bone formation with negligible loss in the volume or integrity of the tissue that receives the implant. See also U.S. patent application Ser. No. 08/030,709, filed Mar. 13, 1993. The material of Brown and Chow has been employed, e.g., to reconstruct two centimeter diameter calvarial defects in cats. The calcium phosphate cement was gradually replaced by bone, instead of being fully resorbed without bone deposition or remaining as a permanent implant. See Chow, Takagi, Costantino and Friedman, "Self-Setting Calcium Phosphate Cements," 179 Mat. Res. Soc. Symp. Proc. 3 (1991). A virtually identical calcium phosphate system which consists of tetracalcium phosphate and monocalcium phosphate or its monohydrate form was described by Constantz et al. (Compare U.S. Pat. Nos. 5,053,212; 5,129,905 and 5,178,845). This cement system is believed to involve, in some embodiments, conversion of the monocalcium phosphate to dicalcium phosphate which reacts with tetracalcium phosphate to form hydroxyapatite.